The present invention relates to magnetic head and disk testers, and more particularly to testers with improved accuracy in positioning a magnetic head with respect to a disk.
A magnetic head and disk tester is an instrument that is used for testing the characteristics of magnetic heads and disks such as signal-to-noise ratio, pulse width and so on. Each tester includes two main assemblies, i.e., an electro-mechanical assembly that performs movements of the head with respect to the disk, and an electronic assembly that is responsible for measurements, calculations, and analysis of the measured data. The electro-mechanical assembly of the tester is known as the spinstand. The spinstand generally simulates the motions of the head with respect to the disk that occur in, for example, a hard disk drive. Whatever the accuracy of the electronic measurement portion of the tester, the results of measurements will also depend upon the positioning accuracy provided by the spinstand.
An exemplary spinstand 5 of a prior-art head and disk tester is shown schematically from a top view in FIG. 1A. The spinstand 5 includes a stationary base element 30 that supports the positioning system and the head 12 and disk 10 to be tested. The disk 10 is supported in a preferably horizontal plane in a manner allowing rotary motion of the disk 10 about a spindle axis perpendicular to that horizontal plane. The spinstand 5 includes a coarse positioning system and a fine positioning system arranged in series to effect controlled movement of head 12 with respect to disk 10. The coarse positioning system positions the magnetic head 12 close to its desired position relative to a magnetic disk 10. In the illustrated form, the coarse positioning system includes a stepper motor 28 affixed to base 30. The stepper motor 28 rotationally drives a lead screw 32 that rotates within bearings 24 and engages a nut 34. Nut 34 is rigidly fixed to a slide 18 so that rotary motion of lead screw 32 effects linear motion of slide 18 along guides (not shown) with respect to base element 30, along a translation axis X, or X-axis.
The fine positioning system of spinstand 5 resides on slide 18 and effects relatively minor positional changes to the position of head 12 illustrated by the slide 18. In the illustrated form, the fine positioning system includes a piezo actuator 26 that is disposed between a stop 36 that is rigidly mounted on slide 18 and a deformable (in the direction of x-axis) body 16 also mounted on slide 18. Two bolts 22a and 22b are screwed into deformable body 16 through openings in the stop 36. Piezo actuator 26 is preloaded by springs 20a, 20b that are compressed between the heads of the bolts 22a, 22b and the stop 36. The deformable body 16 at its base is rigidly coupled to slide 18. The top of body 16 is moveable, in response to the piezo actuator 26, supports arm 14, which in turn supports head 12. Arm 14 is coupled to link 16a by a shaft 25. Body 16 functions as a parallel-link mechanism that is sensitive to the expansions and contractions of piezo actuator 26 to small linear displacements (e.g., 0.001 in) for head 12, (relative to disk 10, as supported on base 30) in addition to the major displacements effected by the coarse positioning system.
FIG. 1B shows side view of an exemplary form of deformable body 16 in the system of FIG. 1A. In this form, the deformable body 16 is a parallelogram-structured deformable body comprised of a top and a bottom rigid links 16a and 16b, disposed in parallel, coupled by two side rigid links 16c and 16d, wherein flexures are at the junction of link pairs to allow for angular displacement of the elements while substantially maintaining the parallelogram integrity of the structure. With this structure the piezo element 26 drives the uppermost, as shown, or the top link 16a of deformable body 16 in the x direction relative to slide 18 (and base 30), whereby the magnetic head 12 to be tested remains substantially at the same height throughout the range of its displacement.
Movements of the link 16a of deformable body 16 are measured by an optical linear encoder 38a, 38b, as shown in FIG. 1A. The optical linear encoder 38 consists of a moveable portion 38a (i.e., a glass scale) that is rigidly attached to the top link 16a of deformable body 16 and a stationary portion 38b (i.e., an optical detector) fixed to base 30. A signal generated by optical detector 38b corresponds to movements of top link 16a of deformable body 16 relative to base 30. That signal corresponds to a sum of the linear displacement established by the steppers motor 28 and by the piezo actuator 34 (together with deformable body 16).
Thus, to achieve high accuracy in linear positioning of head 12 over magnetic disk 10, the positioning process is split into steps of coarse and fine positioning. The coarse positioning is provided, in part, by the rotation of lead screw 32 by stepper motor 28. Rotational movement of lead screw 32 is translated into a linear movement of slide 18 by nut 34. Upon completion of coarse positioning, fine positioning is activated by applying a voltage to piezo actuator 26 from an external power supply (not shown). In a manner known in the art, under the effect of the voltage, actuator 26 changes its linear dimension in proportion to the level of the applied voltage. As a result, the top link 16a together with arm 14 and a magnetic head 12 is shifted with respect to magnetic disk 10 in the X direction. The displacement of magnetic head 12 is measured by optical linear encoder 38 and sent to a feedback circuit (not shown) to control the amount of displacement of the deformable body 16, in a manner known in the art.
During the testing, when the top link 16a of deformable body 16 moves arm 14 with magnetic head 12 mounted thereto, an optical linear encoder 38 is used to determine the position of magnetic head 12. In the prior art, the displacement measured by optical linear encoder 38 is considered to be substantially the same as the displacement of the magnetic head 12. However, in practice, the top link 16a of the deformable body 16 may experience yaw (i.e. rotational displacement about an axis perpendicular to the nominal (horizontal) plane of allowed movement) during the movement. Yaw can occur due to different (asymmetrical) stiffness of the weakened portions (i.e. the flexures) of the deformable body 16, or due to different stiffnesses of the springs 20a and 20b. FIG. 2 shows the effect of the parallelogram-structured deformable body 16 rotating about a point O in the direction indicated by arrow A. As shown, the head 12 moves from an original point P to a point Q. This movement corresponds to a shift X.sub.1 in the X direction, and to a shift Y.sub.1, in the Y direction. Optical linear encoder 38a, 38b can only detect movements in the X direction; in this particular case, it detects, a movement of X.sub.2, which is not equal to X.sub.1. The difference X.sub.1 -X.sub.2 and the shift Y.sub.1, cannot be compensated by the normal, prior art feedback circuit, since the yaw component is undetectable. Therefore, the prior art spinstand 5 shown in FIGS. 1A and 1B cannot achieve very high positioning accuracy.
This problem of accuracy is solved to some degree in a prior art disk and head tester designated as Model 1701, developed and manufactured by Guzik Technical Enterprises, San Jose, Calif. This tester uses a high-precision micropositioning mechanism that performs fine movements. Although this mechanism operates very efficiently and is advantageous for some applications, it is expensive to manufacture because it requires the use of many interacting parts, relative to, for example, the tester of FIGS. 1A, 1B, and 2.
Another disadvantage of the prior art spinstand shown in FIGS. 1A, 1B, and 2 is that the parallelogram-structured deformable body 16, the arm 14, and the head 12 tend to oscillate in the direction indicated by arrow A when the piezo actuator 26 changes its length. The reason for this is that the center of mass of the combination consisting of deformable body 16, arm 14, and magnetic head 12 is not on the longitudinal axis of piezo actuator 26. As a result, this configuration increases the settling time of magnetic head 12 (the time that is necessary to move magnetic head 12 from one point to another).
It is, accordingly, an object of the present invention to provide a magnetic head and disk tester, with relatively few parts, that ensures high accuracy of positioning of a magnetic head over a magnetic disk by compensating for yaw. It is yet another object of the present invention is to decrease the settling time of a head and disk tester.